Many modern analog circuits include power management systems that are arranged to provide protection and performance functions. One protection function is referred to as over-current protection. Over-current protection is typically required when a short-circuit condition occurs in the output of a circuit such as a voltage regulator. The over-current protection function can be accomplished by monitoring the current delivered to a load, and clamping the current when the system detects that it has exceeded some maximum level.
An example voltage regulator with over-current protection circuit (100) is illustrated in FIG. 1. Circuit system 100 includes seven p-type field effect transistors (FETs P0–P6), two n-type field effect transistors (FETs N1–N2), an error amplifier, seven resistors (R1–R7), a capacitor (Co), and two switches (SW1, SW2).
During operation, circuit 100 is operated from a VCC power supply. Reference currents (IREF1, IREF2) are applied to diode connected transistors P3 and P6 respectively. A reference voltage (VREF) is applied to the inverting input of the error amplifier. Resistors R3 and R4 are connected to the output load (R5, Co) to provide a feedback voltage to the non-inverting input of the error amplifier. Transistor P0 delivers an output current (IOUT) to the load (R5, Co), which is replicated by transistor P1. Transistor P3 senses the output voltage to bias transistor P4. Transistors P5 and P6 are arranged to provide a short circuit threshold voltage (VSC) to the gate of transistor P2 and the control terminal of switch SW1. During normal operation, the feedback voltage from resistors R3 and R4 is used to regulate the output voltage (VOUT) across the load (R5, Co) by limiting the output current level (IOUT) via control of the gate voltage of transistor P0.
A short-circuit condition is simulated by the closing of a switch (SW2), which shorts the load (R5, Co) to the circuit ground. Transistor P3 senses the collapsing output voltage from the short-circuit condition and changes the operating current of transistor P4. Transistor N1 senses the current of transistors P1 and P4 and pulls down (via transistor N2) on the short-circuit threshold voltage (VSC) such that transistor P2 is enabled. The regulator output current (IOUT) is clamped to a maximum level at 3 times of its maximum load current normally.